[wasm] Enforce strict ordering of WASM module sections.

src/wasm/encoder.cc

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/signature.h"
 
 #include "src/handles.h"
 #include "src/v8.h"
 #include "src/zone-containers.h"
 
@@ skipping 36 matching lines @@
 
 void EmitUint32(byte** b, uint32_t x) {
   WriteUnalignedUInt32(*b, x);
   *b += 4;
 }
 
 // Sections all start with a size, but it's unknown at the start.
 // We generate a large varint which we then fixup later when the size is known.
 //
 // TODO(jfb) Not strictly necessary since sizes are calculated ahead of time.
-const size_t padded_varint = 5;
+const size_t kPaddedVarintSize = 5;
 
 void EmitVarInt(byte** b, size_t val) {
   while (true) {
     size_t next = val >> 7;
     byte out = static_cast<byte>(val & 0x7f);
     if (next) {
       *((*b)++) = 0x80 | out;
       val = next;
     } else {
       *((*b)++) = out;
       break;
     }
   }
 }
 
 size_t SizeOfVarInt(size_t value) {
   size_t size = 0;
   do {
     size++;
     value = value >> 7;
   } while (value > 0);
   return size;
 }
 
 void FixupSection(byte* start, byte* end) {
   // Same as EmitVarInt, but fixed-width with zeroes in the MSBs.
-  size_t val = end - start - padded_varint;
+  size_t val = end - start - kPaddedVarintSize;
   TRACE("  fixup %u\n", (unsigned)val);
-  for (size_t pos = 0; pos != padded_varint; ++pos) {
+  for (size_t pos = 0; pos != kPaddedVarintSize; ++pos) {
     size_t next = val >> 7;
     byte out = static_cast<byte>(val & 0x7f);
-    if (pos != padded_varint - 1) {
+    if (pos != kPaddedVarintSize - 1) {
       *(start++) = 0x80 | out;
       val = next;
     } else {
       *(start++) = out;
       // TODO(jfb) check that the pre-allocated fixup size isn't overflowed.
     }
   }
 }
 
 // Returns the start of the section, where the section VarInt size is.
 byte* EmitSection(WasmSection::Code code, byte** b) {
+  // Emit a placeholder for the length.
   byte* start = *b;
-  const char* name = WasmSection::getName(code);
-  size_t length = WasmSection::getNameLength(code);
-  TRACE("emit section: %s\n", name);
-  for (size_t padding = 0; padding != padded_varint; ++padding) {
+  for (size_t padding = 0; padding != kPaddedVarintSize; ++padding) {
     EmitUint8(b, 0xff);  // Will get fixed up later.
   }
+  // Emit the section name.
+  const char* name = WasmSection::getName(code);
+  TRACE("emit section: %s\n", name);
+  size_t length = WasmSection::getNameLength(code);
   EmitVarInt(b, length);  // Section name string size.
   for (size_t i = 0; i != length; ++i) EmitUint8(b, name[i]);
+
   return start;
 }
 }  // namespace
 
 struct WasmFunctionBuilder::Type {
   bool param_;
   LocalType type_;
 };
 
 
@@ skipping 424 matching lines @@
   size_t body_size;
 
   size_t total() { return header_size + body_size; }
 
   void Add(size_t header, size_t body) {
     header_size += header;
     body_size += body;
   }
 
   void AddSection(WasmSection::Code code, size_t other_size) {
-    Add(padded_varint + SizeOfVarInt(WasmSection::getNameLength(code)) +
+    Add(kPaddedVarintSize + SizeOfVarInt(WasmSection::getNameLength(code)) +
             WasmSection::getNameLength(code),
         0);
     if (other_size) Add(SizeOfVarInt(other_size), 0);
   }
 };
 
 WasmModuleIndex* WasmModuleWriter::WriteTo(Zone* zone) const {
   Sizes sizes = {0, 0};
 
   sizes.Add(2 * sizeof(uint32_t), 0);  // header
 
-  sizes.AddSection(WasmSection::Code::Memory, 0);
-  sizes.Add(kDeclMemorySize, 0);
-  TRACE("Size after memory: %u, %u\n", (unsigned)sizes.header_size,
-        (unsigned)sizes.body_size);
-
   if (globals_.size() > 0) {
     sizes.AddSection(WasmSection::Code::Globals, globals_.size());
     /* These globals never have names, so are always 3 bytes. */
     sizes.Add(3 * globals_.size(), 0);
     TRACE("Size after globals: %u, %u\n", (unsigned)sizes.header_size,
           (unsigned)sizes.body_size);
   }
 
   if (signatures_.size() > 0) {
     sizes.AddSection(WasmSection::Code::Signatures, signatures_.size());
     for (auto sig : signatures_) {
       sizes.Add(
           1 + SizeOfVarInt(sig->parameter_count()) + sig->parameter_count(), 0);
     }
     TRACE("Size after signatures: %u, %u\n", (unsigned)sizes.header_size,
           (unsigned)sizes.body_size);
   }
 
   if (functions_.size() > 0) {
     sizes.AddSection(WasmSection::Code::Functions, functions_.size());
     for (auto function : functions_) {
       sizes.Add(function->HeaderSize() + function->BodySize(),
                 function->NameSize());
     }
     TRACE("Size after functions: %u, %u\n", (unsigned)sizes.header_size,
           (unsigned)sizes.body_size);
   }
 
+  if (indirect_functions_.size() > 0) {
+    sizes.AddSection(WasmSection::Code::FunctionTable,
+                     indirect_functions_.size());
+    for (auto function_index : indirect_functions_) {
+      sizes.Add(SizeOfVarInt(function_index), 0);
+    }
+    TRACE("Size after indirect functions: %u, %u\n",
+          (unsigned)sizes.header_size, (unsigned)sizes.body_size);
+  }
+
+  sizes.AddSection(WasmSection::Code::Memory, 0);
+  sizes.Add(kDeclMemorySize, 0);
+  TRACE("Size after memory: %u, %u\n", (unsigned)sizes.header_size,
+        (unsigned)sizes.body_size);
+
   if (start_function_index_ >= 0) {
     sizes.AddSection(WasmSection::Code::StartFunction, 0);
     sizes.Add(SizeOfVarInt(start_function_index_), 0);
     TRACE("Size after start: %u, %u\n", (unsigned)sizes.header_size,
           (unsigned)sizes.body_size);
   }
 
   if (data_segments_.size() > 0) {
     sizes.AddSection(WasmSection::Code::DataSegments, data_segments_.size());
     for (auto segment : data_segments_) {
       sizes.Add(segment->HeaderSize(), segment->BodySize());
     }
     TRACE("Size after data segments: %u, %u\n", (unsigned)sizes.header_size,
           (unsigned)sizes.body_size);
   }
 
-  if (indirect_functions_.size() > 0) {
-    sizes.AddSection(WasmSection::Code::FunctionTable,
-                     indirect_functions_.size());
-    for (auto function_index : indirect_functions_) {
-      sizes.Add(SizeOfVarInt(function_index), 0);
-    }
-    TRACE("Size after indirect functions: %u, %u\n",
-          (unsigned)sizes.header_size, (unsigned)sizes.body_size);
-  }
-
   if (sizes.body_size > 0) {
     sizes.AddSection(WasmSection::Code::End, 0);
     TRACE("Size after end: %u, %u\n", (unsigned)sizes.header_size,
           (unsigned)sizes.body_size);
   }
 
   ZoneVector<uint8_t> buffer_vector(sizes.total(), zone);
   byte* buffer = &buffer_vector[0];
   byte* header = buffer;
   byte* body = buffer + sizes.header_size;
 
   // -- emit magic -------------------------------------------------------------
   TRACE("emit magic\n");
   EmitUint32(&header, kWasmMagic);
   EmitUint32(&header, kWasmVersion);
 
-  // -- emit memory declaration ------------------------------------------------
-  {
-    byte* section = EmitSection(WasmSection::Code::Memory, &header);
-    EmitVarInt(&header, 16);  // min memory size
-    EmitVarInt(&header, 16);  // max memory size
-    EmitUint8(&header, 0);    // memory export
-    static_assert(kDeclMemorySize == 3, "memory size must match emit above");
-    FixupSection(section, header);
-  }
-
   // -- emit globals -----------------------------------------------------------
   if (globals_.size() > 0) {
     byte* section = EmitSection(WasmSection::Code::Globals, &header);
     EmitVarInt(&header, globals_.size());
 
     for (auto global : globals_) {
       EmitVarInt(&header, 0);  // Length of the global name.
       EmitUint8(&header, WasmOpcodes::MemTypeCodeFor(global.first));
       EmitUint8(&header, global.second);
     }
@@ skipping 23 matching lines @@
   if (functions_.size() > 0) {
     byte* section = EmitSection(WasmSection::Code::Functions, &header);
     EmitVarInt(&header, functions_.size());
 
     for (auto func : functions_) {
       func->Serialize(buffer, &header, &body);
     }
     FixupSection(section, header);
   }
 
+  // -- emit function table ----------------------------------------------------
+  if (indirect_functions_.size() > 0) {
+    byte* section = EmitSection(WasmSection::Code::FunctionTable, &header);
+    EmitVarInt(&header, indirect_functions_.size());
+
+    for (auto index : indirect_functions_) {
+      EmitVarInt(&header, index);
+    }
+    FixupSection(section, header);
+  }
+
+  // -- emit memory declaration ------------------------------------------------
+  {
+    byte* section = EmitSection(WasmSection::Code::Memory, &header);
+    EmitVarInt(&header, 16);  // min memory size
+    EmitVarInt(&header, 16);  // max memory size
+    EmitUint8(&header, 0);    // memory export
+    static_assert(kDeclMemorySize == 3, "memory size must match emit above");
+    FixupSection(section, header);
+  }
+
   // -- emit start function index ----------------------------------------------
   if (start_function_index_ >= 0) {
     byte* section = EmitSection(WasmSection::Code::StartFunction, &header);
     EmitVarInt(&header, start_function_index_);
     FixupSection(section, header);
   }
 
   // -- emit data segments -----------------------------------------------------
   if (data_segments_.size() > 0) {
     byte* section = EmitSection(WasmSection::Code::DataSegments, &header);
     EmitVarInt(&header, data_segments_.size());
 
     for (auto segment : data_segments_) {
       segment->Serialize(buffer, &header, &body);
     }
     FixupSection(section, header);
   }
 
-  // -- emit function table ----------------------------------------------------
-  if (indirect_functions_.size() > 0) {
-    byte* section = EmitSection(WasmSection::Code::FunctionTable, &header);
-    EmitVarInt(&header, indirect_functions_.size());
-
-    for (auto index : indirect_functions_) {
-      EmitVarInt(&header, index);
-    }
-    FixupSection(section, header);
-  }
-
   if (sizes.body_size > 0) {
     byte* section = EmitSection(WasmSection::Code::End, &header);
     FixupSection(section, header);
   }
 
   return new (zone) WasmModuleIndex(buffer, buffer + sizes.total());
 }
 
 
 std::vector<uint8_t> UnsignedLEB128From(uint32_t result) {
   std::vector<uint8_t> output;
   uint8_t next = 0;
   int shift = 0;
   do {
     next = static_cast<uint8_t>(result >> shift);
     if (((result >> shift) & 0xFFFFFF80) != 0) {
       next = next | 0x80;
     }
     output.push_back(next);
     shift += 7;
   } while ((next & 0x80) != 0);
   return output;
 }
 }  // namespace wasm
 }  // namespace internal
 }  // namespace v8